The present invention relates to a composite laminated structure comprising a substrate and an apertured layer joined together by means of a layer of an adhesive composition. The laminated structure is made extensible in designated zones by mechanical straining and preferably constitutes side flaps in a disposable absorbent article such as a sanitary napkin.
Absorbent articles such as sanitary napkins, pantiliners, and incontinence pads are devices that are typically worn in the crotch region of an undergarment. These devices are designed to absorb and retain liquid and other discharges from the human body and to prevent body and clothing soiling. Sanitary napkins are a type of absorbent article worn by women in a pair of panties and is normally positioned between the wearer""s legs, adjacent to the perineal area of the body. Sanitary napkins in particular with side wrapping elements, often also referred to as side flaps or wings, are disclosed in the literature and are available in the marketplace.
Sanitary napkins having wings or side flaps of various types are disclosed in U.S. Pat. Nos. 4,687,478, 4,608,047, 4,589,876, Reexamination B1 4,589,876, 4,285,343. Sanitary napkins having wings are commonly viewed as providing good protection against soiling.
However, some women find applying sanitary napkins having side flaps to be inconvenient for various reasons. For instance, some women find it to be difficult to attach the side flaps to the underside of the crotch of their panties. This can be due to factors such as difficulties in folding the side flaps properly into place and to stick them to the undergarment As a result, some women still prefer a sanitary napkin without side flaps. In addition, some women who generally prefer a sanitary napkin with side flaps, occasionally (such as during periods of light flow) prefer a sanitary napkin without. Therefore, there is a need for a sanitary napkin which provides an alternative to sanitary napkins having conventional side flaps while still providing a similar protection.
Several variations of sanitary napkins having conventional side flaps have been suggested. For example, U.S. Pat. No. 4,911,701 discloses a sanitary napkin having elastic strands for providing a greater convex shape to the body-facing portion of the central absorbent and for enabling adhesive-free placement of the side flaps into the panties. U.S. Pat. No. 4,940,462 discloses a sanitary napkin with longitudinally expandable flaps. The flaps are designed to fold over the exterior of the wearer""s panty and then to expand to conform with the contour of the panties. Further improvements of side wrapping elements with extensible zones have been disclosed in U.S. application Ser. No. 08/124,180 of Sep. 17, 1993; and Ser. No. 08/277,733 of Jul. 20, 1994 and European application 94202252.6 of Aug. 3, 1994.
In these disclosures the extensibility can be provided by a number of different processes. For example the extensible zones can be created by mechanically straining, corrugating, xe2x80x9cring-rollingxe2x80x9d, heating and deforming, subjecting portions of the side wrapping elements or flaps to compression between mating plates, and the like.
In particular high speed mechanical straining such as corrugating or ring-rolling are desirable due to the manufacturing efficiency from allowing high speed production. Also ring-rolled zones of extensibility can have an angled extensibility relative to the machine direction (process transport direction). Suitable methods for ring-rolling, typically by means of two rolls of meshed interlocking teeth and grooves, are described in U.S. Pat. Nos. 4,107,364, 4,834,741, 5,143,679, 5,156,793 and 5,167,897.
Particularly preferred side wrapping elements or flaps comprise at least two layers of material but sometimes more, for example four layers of materials which are joined together forming a composite laminated structure that is rendered extensible only after being formed into a laminate. In particular prior art sanitary napkins with side wrapping elements having extensible zones are constructed such that the side wrapping elements are formed by laterally extending parts of the uppermost layer of the sanitary napkin (typically called topsheet) and the lowermost layer of the sanitary napkin (typically designated the backsheet).
The laminate formed in the area laterally outside the main portion of the sanitary napkin is strained by mechanical processes like the aforementioned ring-rolling at the production speed of these articles. For the laminate to display uniform behaviour during the mechanical straining the individual layers need to be properly joined to each other at least in the areas of mechanical straining.
Typically adhesives, especially hot melt adhesives have been suggested and used for that purpose. A large variety of adhesives for the different situations occurring in the manufacturing process of disposable absorbent articles have been developed and continue to be developed. Alternative joining methods include welding which is used between plastic materials of similar kinds allowing to create areas were the materials are fused to each other to create permanent connections, or crimping which is a local mechanical deformation of the layers such that the layers interlock locally.
A problem encountered when applying mechanical strain, for example by means of a ring rolling process, to a composite laminated structure comprising an apertured layer joined by means of an adhesive to a substrate, in order to provide designated zones of the structure with extensibility, is that machinery parts, typically the apexes of the metal teeth of one of the rolls used in the ring rolling process, come into direct contact with the adhesive layer through the apertures of the apertured layer. The apertured layer can be a nonwoven or, preferably, an apertured three dimensional polymeric film, constituting e.g. the topsheet of a sanitary napkin, and the substrate can be a liquid impermeable polymeric film, preferably a liquid impermeable, moisture vapour permeable layer, such as a microporous breathable film, comprised in a preferably breathable backsheet.
Adhesives used in the field of disposable absorbent articles are tackified to display adhesive characteristics over a broad range of temperatures. They are usually still tacky at room temperature and therefore during the mechanical straining step by means of e.g. ring rolling they can still stick to the teeth of a roll for ring rolling through the apertures of the apertured layer.
It is particularly undesirable during the straining process to have exposed adhesive on the outside of any of the laminates because adhesive build up, in particular sticky adhesive build up on fast moving machinery parts quickly leads to unstable process conditions. As a minimum the occurrence of these conditions require frequent cleaning but can even cause disastrous material destruction leading to machine stops and reduce efficiency. Particularly, when a ring rolling process is performed and one of the layers of the laminated structure has apertures the adhesive that sticks to the apexes of the teeth of one of the rolls can pull the portions of the layer joined to the apertured layer, i.e. the substrate, through the apertures of the apertured layer, to the extent that ruptures and pin holes are created in this substrate. This is detrimental to the integrity of the structure, and also to the liquid imperviousness of the substrate, when, as it is preferred, it constitutes at least part of a backsheet structure in a disposable absorbent article, and therefore ruptures in the substrate ultimately can cause liquid leakage during the use of the product.
This pulling action is even more effective in case of a composite laminated structure comprising a three dimensional apertured thermoplastic polymeric film as the apertured layer and a microporous breathable film as the substrate, since the presence of the micropores in the microporous film makes the film somewhat weaker and more subject to ruptures under the pulling action of the adhesive stuck e.g. to the teeth of a roll for ring rolling.
Adhesive which may be exposed through the apertures of an apertured layer on the outside of a composite laminated structure in a disposable absorbent articles is also highly undesirable for the consumer. If adhesives which are tacky at the usage temperature of these products contact the wearer""s skin or garment they will usually cause residue adhesive on garments or irritation to the wearer""s skin.
These undesirable effects can be also enhanced by the mechanical straining itself, e.g. a ring rolling process of a laminated structure comprising a three dimensional apertured film joined to a substrate, e.g. a microporous breathable film, since the mechanical strain induced by the ring rolling increases the size of the apertures in the three dimensional apertured film, and at the same time at least partially destroys the three dimensional structure of the apertured film itself, so reducing its thickness. This means that the adhesive is exposed through enlarged apertures on the outside of such a laminate, and is also closer to the external surface, and therefore is more likely to come in contact e.g. with the wearer""s skin or garment.
It has been found that an alternative method of joining the materials of a composite laminated structure, which is called soldering, can be used in the process of mechanical straining of the laminated structure. This method as such is known from the art of joining metals, but has been also been applied in the field of disposable absorbent articles, as described in patent applications EP-A-707841 and EP-A-710470. A solder, as described in the cited application, is an adhesive which shows adhesive behaviour when it is applied at its process temperature, but is not sticky at room temperature. Therefore during the mechanical straining step, e.g. by means of ring rolling, of a composite laminated structure comprising a substrate joined to an apertured layer by means of the solder, the adhesive used as the solder cannot stick to the machine parts, and to anything else during the subsequent use of the laminate, as far as the solder (adhesive) is kept at room temperature.
A problem with composite laminated structures comprising an apertured layer joined to a substrate by means of known adhesives used as solders is that such adhesives, while being not sticky at room temperature, and therefore capable of avoiding both the adhesive build up onto the machinery parts, and consequent damages to the laminated structure due to adhesion between the laminated structure itself and the machinery parts onto which the adhesive can stick, and possible stickiness of the laminated structure during the use of the structure itself, for example in a disposable absorbent product, do not have sufficient extensibility to withstand the mechanical straining process.
Known adhesives used as solders in such laminated structures for their preferred behaviour in terms of no stickiness at room temperature have a relatively high hardness and a low elongation at break, and therefore make the composite laminated structure stiff. This causes the adhesive layer joining the layers of the composite laminated structure to break into small pieces when the laminated structure is subjected to mechanical straining, e.g. by means of ring rolling, to be provided with extensibility in designated zones. The joining between the layers of the composite laminated structure can therefore fail and cause delamination of the structure, i.e. separation of the layers over at least part of the joined area.
It is therefore an object of the present invention to provide a composite laminated structure comprising a substrate and an apertured layer joined together by means of an adhesive layer which, while avoiding the problems related to the exposure of the adhesive outside of the structure, e.g. direct contact of the adhesive with machinery parts through the apertures of the apertured layer, also achieves a secure joining of the layers when the structure is mechanically strained in order to be provided with designated zones of extensibility.
It is another object of the present invention to provide a disposable absorbent article comprising the composite laminated structure having extensible zones, wherein the substrate is a liquid impervious, preferably moisture vapour permeable backsheet, and the apertured layer is a liquid permeable topsheet. Preferably the absorbent article is a sanitary napkin with side flaps, and the flaps comprise the composite laminated structure having zones of extensibility to facilitate folding of the flaps.
A composite laminated structure comprising a substrate, an apertured layer bonded to the substrate, and a layer of an adhesive composition comprised therebetween and adhesively joining the substrate and the apertured layer. The composite laminated structure has designated zones rendered extensible by mechanical straining.
The adhesive composition has a peel force on steel at a temperature of 23xc2x0 C. of less than 10 g according to the Peel Force on Steel Test described herein, and the adhesive composition has an elongation at break of at least 35% according to ASTM D 638M-91a where the sample thickness is 4 mm.
The composite laminated structure of the present invention is preferably comprised in a disposable absorbent article, preferably a sanitary napkin. The mechanical straining is preferably provided by ring rolling according to the above identified prior art references.